The estuary
Harvey Estuary is a long, flooded, interdunal depression of 56 km2 area of
the Peel-Harvey Estuarine System, located 75 km south of Perth, Western Australia
(32o 30' latitude 115o 40' longitude). The region has
a Mediterranean climate, with mean temperatures for January 30 oC
and 18 oC in August. During the last few decades the estuarine system
has received large phosphorus loadings from its rural, coastal plain catchment
where agricultural use of superphosphate increased several fold. Previous studies
have identified the importance of nutrient input (especially phosphorus) and
light penetration in controlling the prolific seasonal growth of macroalgae
(Chaetomorpha linum, Ulva rigida, Cladophora montagneana) and microalgae
(Nodularia spumigena) in the system (Lukatelich
and McComb 1986a,b; 1989). The macroalgae were not prolific in Harvey Estuary
where sediment resuspension increased the turbidity of the water, limiting the
growth of benthic primary producers. Massive blooms of the blue-green alga Nodularia
spumigena have occurred in Harvey Estuary since 1978, resulting in depletion
of oxygen and fish kills. It has also been demonstrated that high levels of
phosphorus flux (> 20 mg m2 day-1) can be released from sediments
to the overlying water under anoxic conditions (Lukatelich
and McComb 1986b; McAuliffe 1991).
Sediment phosphorus release increases internal phosphorus loading, and has been
considered a critical factor in the regulation of algal blooms in the estuarine
system.
Sediment collection
Sediments were collected using a 9.3 cm diameter perspex corer, driven by hand 10 cm deep into the sediment. On each occasion approximately 80 cores (average 27 cores at each sites) were taken by divers, and returned to a boat for sub-sampling. The water was carefully extruded upwards out of the core by a hand-driven plunger. Sections of 0-2 cm and 2-5 cm were cut from the cores into a 25 L plastic bucket, returned to the laboratory, and stored at 4oC.
Three sites were sampled in Harvey Estuary (Figure 1). Station 28 situated 100 m from the western edge of the estuary in approximately 1 m of water; station 1, in the central basin overlain by 1.8 m of water and station 27, 50 m from the eastern shore on the peripheral platform of the estuary in approximately 0.5 m of water. These sites form part of a larger sampling programme for the management agency, hence the non-sequential station numbers. The sites used in this study were chosen to represent the central basin and peripheral platforms of the estuary. The properties of sediments, and the fractional composition of sediment phosphorus of the three sites are shown in Table 1 and Table 2.
Reconstituted cores
Sediment sections of known weight were re-packed into the bottom of 60 cm long, transparent PVC cores, and the end of each was sealed with a rubber bung. The 2-5 cm sediment was repacked into the bottom of the cores, followed by 0-2 cm sediment. Sediment cores were gently shaken to force out air bubbles and to level the sediment surface. The surface was carefully overlain with a 50 cm column of filtered (1 µm) sea-water. Each core was approximately 9.3 cm in diameter, giving a water column volume of 3.4 L and a sediment surface area of 68 cm2. Cores were wrapped in black plastic to minimise photosynthesis and maintained at 20 oC + 2 oC in a temperature-controlled room. The reconstituted cores were used instead of using "undisturbed sediment cores" for two reasons: firstly, the large number of sediment cores sampled (80 cores) make it difficult to incubate all cores in laboratory in a time- and cost-effective way. Secondly, previous work in this estuary has shown that nutrients and organic carbon was enriched in top 5 cm of the sediments, thus this layer is of most interest in relation to the purposes of this study.
Core manipulation
Air was pumped through an air-stone 5 to 10 cm above the sediment surface. Water was carefully circulated from bottom to surface by pumps at a approximately 500 mL per minute, giving minimum sediment resuspension. Tubing was wrapped in black plastic to avoid algal growth. During resuspension experiments, wall-mounted domestic food mixers fitted with additional speed controllers were used. Uniformity was achieved by adjusting the height above the sediment surface and consistent revolution, measured by a tachometer. Sucrose 0.3 g (Analytical grade) was added to sediment cores as a bioavailable carbon source to stimulate oxygen demand. The top 2 cm of surface sediment was suspended for four hours at intervals of 3, 5 and 7 days during the resuspension experiment.
Calcium-nitrate tetrahydrate (Ca(NO3)2.4H2O) was used for nitrate addition.
Practically, nitrate may be applied either to the water column or the upper
few cm of sediment. The effects of method of nitrate application on phosphate
release were examined using Station 1 sediment #3. In the 'sediment application',
nitrate was mixed into the top two cm of sediment at the time of core construction.
In the 'water column application' nitrate was added through a small volume of
solution directly into water column. For both methods of application, the cores
received 10 mg L-1, 25 mg L-1 and 35 mg L-1
of NO3-N. The relevant details of these experiments are summarised in Table
3.
Data recording and analysis
Water samples, usually about 50 mL, were taken from the cores using a teflon tube, lowered to just above the sediment, capped and withdrawn. This was repeated until the required volume of water had been collected. Water sampling was carried out after physical measurements (dissolved oxygen, pH and redox potential Eh) which might otherwise have increased mixing of the water.
Dissolved oxygen (DO) was measured using a "Yeokal" dissolved oxygen/temperature meter 5 cm above the sediment surface. After DO measurement a newly-calibrated Beckman model 21 pH meter was used to measure water column pH.
Redox potential (Eh) at the sediment surface was measured by a platinum electrode inserted 5 mm into the sediment. Readings were taken when the variation was less than 1.0 mV in ten seconds. This stability was usually reached in 3 - 5 minutes. Any replicate readings which differed by more than 20 mV were repeated. The instrument was calibrated using Zobell's solution (Zobell 1946) prior to each measurement.
Figure 2 shows the degree of variation between replicate Eh measurements from a special design to test precision of the method.
Nitrate was determined by Technicon Autoanalyser (Grasshoff et al. 1983). Total phosphorus concentration in the water column was determined after sample digestion with 1:1 mixture of nitric and perchloric acids. Phosphate was determined as soluble reactive phosphorus (SRP) by the single solution molybdenum blue method, following 0.45 µm filtration (Major et. al. 1972). Phosphorus fractionation was carried out using the procedures of Williams et al. (1980).